In general, a reciprocating compressor is designed to form a compression space to/from which an operation gas is sucked/discharged between a piston and cylinder, and the piston linearly reciprocates inside the cylinder to compress refrigerants.
Most reciprocating compressors today has a component like a crankshaft to convert a rotation force of a drive motor into a linear reciprocating drive force for the piston, but a problem arises in a great mechanical loss by such motion conversion. This explains why so many linear compressors are being developed.
Linear compressors have a piston that is connected directly to a linearly reciprocating linear motor, so there is no mechanical loss by the motion conversion, thereby not only enhancing compression efficiency but also simplifying the overall structure. Moreover, since their operation is controlled by controlling an input power to a linear motor, they are much less noisy as compared to other compressors, which is why linear compressors are widely used in indoor home appliances such as a refrigerator.
FIG. 1 illustrates one example of a linear compressor in accordance with a prior art. The conventional linear compressor has an elastically supported structure inside a shell (not shown), the structure including a frame 1, a cylinder 2, a piston 3, a suction valve 4, a discharge valve assembly 5, a linear motor 6, a motor cover 7, a supporter 8, a body cover 9, mainsprings S1 and S2, a muffler assembly 10, and an oil feeder 20.
The cylinder 2 is insertedly fixed to the frame 1, and the discharge assembly 5 constituted by a discharge valve 5a, a discharge cap 5b, and a discharge valve spring 5c is installed to cover one end of the cylinder 2. The piston 3 is inserted into the cylinder 2, and the suction valve 4 which is very thin is installed to open or close a suction port 3a of the piston 2.
The linear motor 6 is installed in a manner that a permanent magnet 6c linearly reciprocates while maintaining the air-gap between an inner stator 6a and an outer stator 6b. To be more specific, the permanent magnet 6c is connected to the piston 3 with a connecting member 6d, and an interactive electromagnetic force between the inner stator 6a, the outer stator 6b, and the permanent magnet 6c makes the permanent magnet 6c linearly reciprocating to actuate the piston 3.
The motor cover 7 supports the outer stator 6b in an axial direction to fix the outer stator 6b and is bolted to the frame 1. The body cover 9 is coupled to the motor cover 7, and between the motor cover 7 and the body cover 9 there is the supporter 8 that is connected to the other end of the piston 3, while being elastically supported in an axial direction by the mainsprings S1 and S2. The muffler assembly 10 for sucking in refrigerant is also fastened to the supporter 8.
Here, the mainsprings S1 and S2 consist of four front springs S1 and four rear springs S2 that are arranged in horizontally and vertically symmetrical positions about the supporter 8. As the linear motor 6 starts running, the front springs S1 and the rear springs S2 move in opposite directions and buff the piston 3 and the supporter 8. In addition to these springs, the refrigerant in the compression space P functions as sort of a gas spring to buff the piston 3 and the supporter 8.
Therefore, when the linear motor 6 starts running, the piston and the muffler assembly 10 connected to it move in a linear reciprocating direction, and with the varying pressure in the compression space P the operation of the suction valve 4 and the discharge valve assembly 5 are automatically regulated. Under this mechanism, the refrigerant flows via a suction pipe on the side of the shell, an opening of the body cover 9, the muffler assembly 10, and suction ports 3a of the piston 3 until it is sucked in the compression space P and compressed. The compressed refrigerant then escapes to the outside through the discharge cap 5b, the loop pipe and an outlet duct on the side of the shell.
Having as many as eight mainsprings S1 and S2, the linear compressor is now faced not only with a large number of springs but also with many variables that need to be controlled to stay balanced during the motion (e.g., pumping stroke) of the piston 3. These pose certain problems such as complicated and lengthy manufacturing process and increased manufacturing costs.
Moreover, the conventional linear compressor is designed in such a way that its resonance frequency is synchronized with an operating frequency of the linear motor 6 to increase compression efficiency. Here, the resonance frequency varies by many factors, e.g., stiffness of the mainsprings S1 and S2, stiffness of a gas spring, a total mass of immovable members including a cylinder (hereinafter, they are referred to as fixed members), and a total of mass of members that are operationally coupled with the piston 3 (hereinafter, they are referred to as movable members), so one can easily synchronize the resonance frequency with the operating frequency, simply by increasing the mass of the movable member. One typical way to increase the mass to the movable member is to fasten a mass member (not shown) to the piston 3 and the connecting member 6d on the side of the supporter 8, but because of the newly added mass member, initial positions of the front and rear springs S1 and S2 shift and so does the initial position of the piston 3 with respect to the cylinder 2. In result, volume of the compression space changes, thereby changing stiffness of the gas spring at the same time. That is, when someone changes a total mass of the movable member trying to synchronize the resonance frequency with the operating frequency, wanted or not, stiffness of the gas spring varies and this in turn makes it hard to synchronize the resonance frequency with the operating frequency. Therefore, a problem situation is created where it is difficult to manage running conditions of the linear compressor as efficiently as possible, the compression efficiency is degraded, the manufacturing process becomes more complicated as additional changes have to be made in design, and the shorter stroke length of the piston 3 also impairs an overall compression efficiency.
FIG. 2 is a side sectional view of a conventional linear compressor, and FIG. 3 is a longitudinal sectional view of FIG. 2.
Referring to FIG. 2, a conventional linear compressor 1 is configured that inside a hermetic shell 10, a piston 30, which is driven by a linear motor 40, linearly reciprocates inside a cylinder 20 to suck in, compress and discharge refrigerant. The linear motor 40 includes an inner stator 42, an outer stator 44, and a permanent magnet 46 positioned between the inner stator 42 and the outer stator 44. Here, the permanent magnet 46 is driven by an interactive electromagnetic force to make a linear reciprocating motion. As the permanent magnet 46 moves while being connected to the piston 30, the piston 30 also makes a linear reciprocating motion inside the cylinder 20, thereby sucking in, compressing and discharging refrigerant.
The linear compressor 1 further includes a frame 52, a stator cover 54, and a back cover 56. For the linear compressor, the cylinder 20 may be fastened to the frame 52, or the cylinder 20 and the frame 52 may be integrately formed as well. In front of the cylinder 20, there is a discharge valve 62 which is elastically supported by an elastic member and goes to an open position or to a closed position selectively by pressure of the refrigerant inside the cylinder 20. Moreover, a discharge cap 64 and a discharge muffler 66 which are seated in front of the discharge valve 62 are fastened to the frame 52. One end of the inner stator 42 and one end of the outer stator 44 are also supported on the frame 52. The other end of the inner stator 42 is supported by a separate member such as an O-ring or by a fixed jaw on the cylinder 20, while the other end of the outer stator 44 is supported by the stator cover 54. The back cover 56 is seated on the stator cover 54, and a suction muffler 70 is placed between the back cover 56 and the stator cover 54.
The supporter 32 is coupled to the rear side of the piston 30. The supporter 32 is provided with mainsprings 80, each mainspring having a natural frequency to help the piston 30 resonate. The mainsprings 80 are divided into front springs 82 both ends of which are supported on the supporter 32 and the stator cover, respectively; and rear springs 84 both ends of which are supported on the supporter 32 and the back cover 56, respectively.
FIG. 3 illustrates that four front springs 82 and four rear springs 84 are arranged facing each other. To be more specific, the front springs 82 have two springs on the top and two springs on the bottom, facing each other; and the rear springs 84 have two springs on the left side and two springs on the right side, facing each other. The front springs 82 are supportably mounted between the supporter 32 and the stator cover 54, and the rear springs 84 are supportably mounted between the supporter 32 and the back cover 56.
As discussed earlier, having a large number of mainsprings 80 means that there are going to be a lot of position variables to be controlled to stay balanced during the motion of the piston 30. Because of that, an overall manufacturing process gets complicated and lengthy, and manufacturing costs are high.
Meanwhile, if the suction muffler 70 is secured to the supporter 32 by means of a fastening member, no escape structure is provided for the fastening member. This actually causes an unnecessary interference or friction of the mainsprings to give rise to problems like noise and damages on the mainsprings. Therefore, there is a need to develop a way out for an elaborate elastic motion of the mainsprings.